🦴 Calcium Homeostasis - Hormonal Regulation (Guyton)

Normal serum calcium = 9.4 mg/dL (2.4 mmol/L) Only 0.1% of total body calcium is in ECF - bones are the giant reservoir (99%).

📊 Forms of Calcium in Plasma (Memorize This!)

Key exam point: Ionized Ca²+ = 1.2 mmol/L = the only physiologically active form. Low albumin → low total Ca but normal ionized Ca (patient has no symptoms).

🔑 The Three Hormones - Quick Overview

1️⃣ PARATHYROID HORMONE (PTH) - The Most Important!

PTH Actions - "BKG" (Bone, Kidney, Gut - indirect)

🦴 On BONE:

• Rapid phase (within minutes): Activates existing osteoclasts → pumps Ca²+ and HPO4 out of bone fluid into ECF
• Slow phase (days-weeks): Stimulates osteoclast proliferation (via osteoblast RANKL → activates osteoclasts) → massive bone resorption
• Net result: ↑ Ca²+ AND ↑ PO4 released from bone

🫘 On KIDNEY (most important for phosphate):

1. ↑ Ca²+ reabsorption - in distal tubule and loop of Henle
2. ↓ PO4 reabsorption - in proximal tubule (phosphaturic effect) → PTH LOWERS serum phosphate despite releasing it from bone
3. ↑ 1α-hydroxylase activity → converts 25-OH Vit D → active Calcitriol (1,25-(OH)₂ D₃)

Net kidney effect: Ca²+ retained, PO4 excreted - so net serum result = ↑ Ca²+, ↓ PO4

Gut (INDIRECT):

• PTH → activates Vit D → Vit D ↑ intestinal Ca²+ absorption

Why PTH Lowers Phosphate Despite Bone Resorption:

2️⃣ VITAMIN D (Calcitriol) - The Gut Hormone

Activation Steps (must know!):

Dietary Vit D₃ / Skin UV
        ↓
Liver: 25-hydroxylase → 25-(OH) Vit D (calcidiol) [storage form]
        ↓ (rate-limiting step - stimulated by PTH and ↓PO4)
Kidney: 1α-hydroxylase → 1,25-(OH)₂ Vit D₃ (CALCITRIOL) [active form]

Calcitriol Actions:

Guyton key point: Vitamin D is the ONLY hormone that significantly increases intestinal calcium absorption. Without it, even high Ca diet won't get absorbed properly.

Regulation of Calcitriol synthesis:

• ↑ Production when: PTH ↑, low PO4, low Ca, growth hormone, estrogen
• ↓ Production when: high Calcitriol itself (negative feedback), high PO4

3️⃣ CALCITONIN - The "Emergency Brake"

Actions:

1. Bone: ↓ osteoclast activity → ↓ bone resorption → ↓ Ca²+ and PO4 release
2. Kidney: Minor ↑ Ca²+ and PO4 excretion

Guyton's view: Calcitonin is much less important than PTH in long-term calcium regulation. It mainly acts as a short-term "emergency" response after a large calcium meal. In adults, even removal of the thyroid (losing calcitonin) causes minimal calcium disturbance. PTH is the dominant long-term controller.

🔄 The Feedback Loop (Master This Diagram Logic)

↓ Serum Ca²+
    ↓
PTH secretion ↑↑
    ↓ ────────────────────────────────────────────
    ├── BONE: osteoclasts activated → Ca²+ released
    ├── KIDNEY: Ca²+ reabsorbed, PO4 excreted
    └── KIDNEY: Activates Vit D
                    ↓
              INTESTINE: Ca²+ absorbed ↑
    ↓
Serum Ca²+ returns to normal
    ↓
PTH secretion ↓ (negative feedback)

⚡ Clinical Correlates (Exam Favourites)

🧠 High-Yield One-Liners for MCQs

• Only PTH causes hypercalcemia with hypophosphatemia
• Only PTH activates 1α-hydroxylase in kidney → activates Vit D
• Ionized calcium controls PTH secretion, NOT total calcium
• Albumin change: For every 1 g/dL drop in albumin, total Ca drops ~0.8 mg/dL (but ionized Ca unchanged)
• Bone has two calcium pools: Rapidly exchangeable (ECF-bone equilibrium, not hormone-dependent) and slowly exchangeable (hormone-dependent)
• Calcitonin is most important in children/young adults; less important in adults
• Phosphate and calcium have reciprocal relationship: Ca × PO4 = constant (if one goes up, other tends to fall)

🧪 Normal Values to Memorize

Based on Guyton and Hall Textbook of Medical Physiology (Chapter 80 - Parathyroid Hormone, Calcitonin, Calcium and Phosphate Metabolism, Vitamin D, Bone, and Teeth)